LM317 with shortcut protection diodes schematics

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erqruiozzot

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Can anyone, please explain me how exactly these two diodes protect the circuit and why C2 in series with D2?

What would happen in case of input short circuit (2 to GND) and without D1?

Do these diodes protect the LM itself or the device connected to the output?
 
D2 is to protect the ADJ pin by the C2 capacitor discharge.

If input shorted there will be no output voltage.

D1 is to protect the LM output if you are using a bigger cap on the output.
 
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All ICs have internal substrate diodes that turn on whenever any pin is pulled to a polarity opposite it's normal polarity. When the internal diodes turn on it usually latches up the circuit and it blows up from large current. The external diodes turn on at a lower voltage than the internal ones and prevent the latch up.
 
Gayan Soyza said:
D2 is to protect the ADJ pin by the C2 capacitor discharge.

If input shorted there will be no output voltage.

D1 is to protect the LM output if you are using a bigger cap on the output.
Click to expand...

I agree.
(D1) If the input is shorted out, after running for a while, power from the output capacitor will flow backwards through the regulator into the short. The regulator can handle this if the output cap is small. With large output caps the regulator will die. The input to output diode protects against input shorts.

(D2) is not needed if C2 is small.
 
Gayan Soyza said:
D2 is to protect the ADJ pin by the C2 capacitor discharge.
Click to expand...
You mean, without D2, C2 connected to ADJ would destroy the LM? But, there isn't C2 on the original schematics (without short circuit protection), so the question arises - what is the purpose of C2 :-/

Gayan Soyza said:
If input shorted there will be no output voltage.
Click to expand...
So, without the protection diodes, input shorcut would cause only no output voltage and nothing else? But the note under the schematics says: D1 protexts the device against input shortcut - why protection if the only consequence would be no output voltage?

Gayan Soyza said:
D1 is to protect the LM output if you are using a bigger cap on the output.
Click to expand...
The note says: D2 protects against output shortcut - not D1 :-/
 
So the protection would be against the reverse polarity? But the application note says it's against the shotr circuit(s) :-/
 
I think this would be the exact explanation. From the **broken link removed** , seems that is the point of the D1.

ronsimpson said:
(D2) is not needed if C2 is small.
Click to expand...
You mean, if C2 is removed - there is no need for D2? So why would we put C2 in the first place? There are only C1 and C3 on the original schematich (without protection diodes).?
 
I think you are looking at the datasheet from ST Micro who are Italian. Their text does not explain much in English.

National Semi invented the LM317 so you should look at their datasheet where everything is explained in English with details.
National Semi explains the function of C2 and the functions of the diodes on their dsatasheet.
 
Arrghh ))) Thank you very much! The schematics from my first post was in fact from the SGS Thompson datasheet but in the same folder I have the following LM datasheets:

LM317 - Fairchild
LM317 - National Semiconductor
LM317 - ON Semiconductor
LM317k - SGS Thompson
LM317L - ST Microelectronics

and I have even marked the National Semiconductor's datasheet as the best but have overlooked the explanation!!

Thanks to everyone who answered the question.
 

You people are making this way too complicated. I gave the correct answer a long time ago:

All ICs have internal substrate diodes that turn on whenever any pin is pulled to a polarity opposite it's normal polarity.
Click to expand...

When you short the input to ground, the output cap holds a voltage for a while so the input-to-output voltage is reversed with the output being higher than the input: the substrate diode (inherent in the power transistor) turns on and the output cap discharges into the chip which can cause death.

Exact same thing for the cap on the adjust pin. It holds the voltage up as the IC voltages come down, internal diode turns on and cap discharges. Peak discharge current damages IC.

Protect diodes provide parallel path for capacitors to discharge through.

Ain't rocket science.
 
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Audiguru is correct. Here are the two datasheets. Thompson first. And then the updated original from National. I see Surface mount is there too now.

Cheers,
tvtech
 
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National data sheet says its junction temperature can withstand upto 0'C-125'C.I'm wondering when its thermal shutdown starts switches ON?Over 125'C?
 
Gayan Soyza said:
National data sheet says its junction temperature can withstand upto 0'C-125'C.I'm wondering when its thermal shutdown starts switches ON?Over 125'C?
Click to expand...
DO NOT operate an LM317 at its maximum allowed temperature. If it reaches such a high temperatrure many times then the chip might fracture from "thermal fatigue". It expands when hot and shrinks when cooled over and over.
 
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